Dancer roll assembly



Feb. 21, 1961 J. F. PFEFFER, JR

DANCER ROLL ASSEMBLY 2 Sheets-Sheet 1 Filed Oct. 21, 1957 FIG. lb

FIGIC FIG. Io

m m v m JAMES F. PFEFFER, JR.

ATTORNEY 2'0 4'0 ROLL SURFACE FINISH -Rus 2 x 8:25 8522.: it 52321; 52

(.N a cog Feb. 21, 1961 J. F. PFEFFER, JR 2,972,

DANCER ROLL ASSEMBLY Filed Oct. 21, 1957 2 Sheets-Sheet 2' FIG.5

56 OUTSIDE RADIUS OF DANCER ROLL IO SHOULD EQUAL,WITHIN 3O /o 3 THE RADIUS OF GYRATION OF THE SUB-ASSEMBLY CONSISTING OF ROLL IO AND LINK SUPPORT 27-MOUNT 3O ATTORNEY I I 6. 4 I k w x I I L\ I I l I I I I I I I I I 2 3 I L I INVENTOR JAMES F. PFEFFER,JR.

I I W 2 26 BY 0M7/ PM 2,972,451 DANCER ROLL ASSEMBLY James F. liz'etfer, Jr., Wilmington, Del.,[assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed on. 21, 1957, Ser. No. 691,352

'3 Claims. c1. 242-7s.3

This invention relates to a dancer roll assembly, and particularly to a dancer roll assembly adapted to supply a running web or warp sheet at constant tension on travel away from the assembly.

There are numerous situations wherein it is desirable to supply a running web or warp sheet to processing equipment under conditions of substantially constant tension. One instance is in the supply of a cellophane web to a coating tower wherein it is desired to apply a thin coating of waterproofing material or the like during the rapid transit of the running web through the tower. Cellophane is hydrophyllic and the Wet vmaterial possesses a considerably lower strength than the dry cellophane, for which reason it is desirable to iron out all tension variations and thereafter coat the web under as nearly constant tension conditions as possible.

Tension-variation in a running web results from either a change in the frictional drag on the Web imposedby the processing machinery, which is generally gradual in nature and rather long term referred to the time base, and that due toout-of-roundness of the mill rolls, which is short in duration and more or less periodicin nature. Webs of cellophane and polymeric materials generally have smooth surfaces of low coeflicientof friction, and out-of-roundness in roll formation is particularly characteristic of these materials as comparedwith'other products such as paper or cloth. Apparatus adapted to' supply webs at constant exit tension must accommodate for Variations from both the long term and. short term causes to degrees. at least partially dictated by the characteristics of the material in process per se.

An object of this invention is to provide'a dancer roll assembly adapted to supply a wide variety of running web or Warp sheet materials at substantially constant exit tensions. Other objects of this invention comprise providing a dancer roll assembly for the supply of runing webs or warp sheets at substantiallyconstant exit' tension which is simple in design, low in first cost and which occupies only a small amount of space. The

manner in which these and other objects of this invention are obtained will become clear from the detailed description and the following drawings, in which:

Figs. 1a, 1b and 1c are schematic representations of a dancer roll to which the three force vectors determinative of web or Warp sheet delivery tension are applied individually, Fig. 1a being for static conditions solely, Fig. 1b for linear translation solely and Fig. 1c for dancer roll rotation solely, V p

Fig. 2 is a diagrammatic representation of 'a dancer roll assembly accordingto this invention,

"Fig. 3 is a plot of minimum air film thickness at which a smooth running web or warp sheet slips on the surface of a rotatable roll over which the webis threaded,

Fig. 4 is a side elevation view of a preferredembodiment of dancer roll and supporting stand according to this invention, and

Fig. 5 is atop plan view of the reactor carrying end of the embodiment of Fig. 4."

, 2,972,451 Patented Feb. 21,1951

Generally, the dancer roll assembly according to this I from the radius. of gyration of the subassembly constituting the dancer roll andlink support taken together asan entity. -For economy of words, the term web as hereinafter employed is deemed to include warp sheet as well, since the apparatus of this invention is equally operable on either webs or warp sheets without distinction.

Referring to Fig. 1, the forces acting on a dancer roll can be classified in three general categories. In Figs. 1a, 1b and 10, if the dancer roll is designated as a circle 10 and the left-hand run ofthe web represents the web fed to the dancer roll, point A being a specific point in the entering web, conditions can be analyzed by assuming that the right-hand side of the web is the Web leaving the dancer roll, which is shown as attached to afixed point, since it is assumed that a uniform velocity is desired for the exiting web, i.e., that there is zero acceleration. a

Fig. 1a represents the static condition, which corresponds to a constant entrance and exit velocity of the Web during which point A remains fixed in space, under which circumstancesthe weight W of roll 10v is supported byequal tension force components W/2 applied upwardly'to theleftand right-hand runs of the web. Any change in velocityof the entering web must'accelerate thedancerroll in translation, such as where the entering velocity decreases, and A moves upwardly as indicated in Fig 111, when the mass of dancer roll 10 is. acted'on by a force corresponding to the accelerationintranslation. This-force is of the magnitude Ma/2, where a-is the acceleration in translation, applied equally to the leftand right-hand sides of the Web as indicated by the arrows adjacent thereto, Still assuming that insufiicient web is being supplied from theleft-hand side to the dancer roll 10 of Fig. 1b, a force couple in the direction indicated by the-vector Ia, Fig. 1c, will be applied to' inertia of roll '10 about its neutral axis and R is the external radius of roll 10. 'For the rotational force application, however, the force component in thelefthand or entering run of the web is directed away from roll 10, Whereas the force component for the right-hand run of the web is'directed toward roll 10. -In order to cancel out the effects of force application so that substantially constant tension conditions exist for the web leaving the dancer roll, it is plain that oppositely directed force components Ma/Z of Fig. lb-and lot/2R of Fig. 1c mustbe made precisely equal. Since a=a/R and I for a cylinder; Wr /g, where r is the radius of gyration,

it is clear that Ma 2 a/2 and Ia /2R=Wr a/2R g. I Then, equating the two forces, -W a /2 g='Wr a/2R g; or

R =r, i.e., the external radius of the roll, R, must equal the radius of gyration, r, of the dancer roll assembly in order to balance out'the force components resulting from changes in entering web velocity. f

It will be understood that, while precise the dancer roll radius and the radius of gyration of the 'dancer roll assembly is desirable for complete cancellation of theeffects of velocity change in the web supplied, some tolerance is allowable, especially where light liollow cylindrical dancer rolls are employed and where the supporting link structure is light in mass. Satisfactory resultscan generally be obtained where equality equality of 3 of. R with r iswithin about 30%. In order to match the radius of gyration withthe external radius of the dancer roll, it is preferred .to utilize dancer rolls having the form of thin-walled hollow cylinders; however, it is possible to load even rather thick-walled annuliva compensatory amount by attaching concentric rings of metal as. flywheel masses on the outside opposite ends of the dancer rolls. There is, however, a limit of total mass for the assembly, because, if the total mass of the dancer roll is; too great to permit non-slipping rotation by frictional contact with the running web, canceilation of rotational and translational acceleration forces will not occur as represented in the diagrams 1b and 10.

In this connection, intimate contact between the roll and the web is desirable so that no hydrodynamic air film is developed between the web and the roll which results in slippage between the roll and the running web. The thickness of the air film developed by a running film threaded over a roll is given by the equation: h =2R (ZS) /T where:

h =the air film thickness in inches,

R=the external roll radius in inches,

Z=the viscosity as Reynolds units,

S=the web speed in yards/minute, and T,=the web tension per inch of width in lbs./ in.

From this equation it is apparent that the roll radius should be kept small to minimize the air film and that, while the air film decreases in thickness with increase in'web tension, there is a limit on the pull which can be applied to a web due to material strength limitations as well as power requirements. In a typical example where a web was processed at a speed of throughput S of 200 y.p.m. at a tension T of lbs. over a dancer roll 2%" in outside diameter, the calculated air film thickness was 0.00045. The maximum permissible air film thicknesses at which slippage occurs between a running web and a guiding roll have been determined experimentally for smooth webs as a function of roll surface finish in root-mean-square units as plotted in Fig. 3. From the plot .it will be apparent that an air film thickness of 0.00045" would be prohibitive at even rather high roll surface roughnesses. 'It has been found that a wide variety of roll surface roughnesses can be obtained by shot-blasting the rolls, which can thereafter be surfacepolished so as not to damage a running web passed in contact therewith. In the interests of conservative design, it is preferred to operate at air film thicknesses relativeto roll surface finishes well below the curve of Fig. 3.

2 shows in schematic representation a dancer roll assembly according to this invention. Dancer roll 10 is'journaled for free rotation at the outer end of a linksupport structure 11 which is pivoted for free rotation about a point 12. The countersupport for the tension applied to the running web in its transit past roi 10 -is provided by a pressurized air cylinder 16 fitted with 'a piston having a piston rod 17 pinned to the link support 11 at 15. The ambient air pressure within cylinder- 16 is maintained at any desired level by adjustment of valve 13 in air line 14 connected to cylinderport 18. His desirable to maintain a substantially zero spring scale as regards cylinder 16 and piston rod 17 and, ma typical installation wherein the web was processed at a velocity in the range of 100-200 yds./minute at a total tension of -025 lbs, a cylinder in, 35" in length, was utilized with the pressure within the cylinder maintained at 3-15 p.s.i. gage. To minimize piston friction a draped diaphragm seal of 20 sq. in. area was utilized, such seal resembling-the design marketed by the Bellofram Corp, Burlington, Mass. With this arrangementand a dancer roll 16) of 2% diameter, the position taken bythe dancer roll in the course of operation was substantially independent of the tension appliedwithin the full range delineated by the limits indicated in Fig. 2 by broken line representation, which coresponded' to a vertical deviation from the horizontal of 3" in either direction for an apparatus having a ,total lever arm 11 of 12" supported by a piston rod 17 pinned 3" to the left of pivot point 12. At the same time, the calculated natural frequency of the apparatus considered as an elastic system was 0.68 for the lower limiting pressure of 3 p.s.i. gage and 0.85 for the upper limiting pressure of 15 p.s.i. gage, which was sufficiently low to' fall outside of the cyclical variations which could be anticipated in the supply of a running web at any speed within the design range of 100400 yds./minute. Consequently, the dancer roll assembly of this invention is stable and equally effective regardless of the position assumed inthe course of operation.

As an approximation, the effective mass of the dancer roll assembly utilized in the determination of any flywheel weights which might be employed to balance the radius of gyration against the external roll radius has been found to be the sum of the roll mass itself plus the mass of the link support structure to a distanceof about two-thirds in from the dancer roll to pivot point 12. Greater precision in proportioning has been found unnecessary unless extremely close tension control in the web is essential. It will be understood, of course, that freeing the exiting web from inertial effects is accomplished only by accentuating the tension variations in the web on the entering side. This is often not objectionable, as in the case of cellophane, for example, where the dry strength is very high as compared with the wet strength, but in any case this is an inseparable factor in the design which must be borne in mind.

It is desirable to provide the dancer ro-ll assembly with means maintaining it in mid-position, such as with the'link support structure-disposed along the horizontal for the design hereinbefore described, so that the assembly will have the necessary freedom of action as regards corrective oscillation either above or below this line. This is especially desirable in view of the zero spring scale characteristic which'is optimum as regards supporting air'cylinder iii-piston 17. Theprovision of a position restoring auxiliary apparatus is'not objectionable because web unwinding systems require speed'range control apparatus to regulate unwinding speed as a function of changing mill rolldiameter anyway. Accordingly, dancerroll 10 can be profitably employed to coin cidentally sense long term frictional drag change in the system and thereby regulate the speed of unwinding proportionately, so as to keep the unwind tension in the web constant as regards this factor. A variety of conventional control auxiliaries are available commercially which can be employedin conjunction with dancer roll 10 for this purpose, an electrical reactor being completely satisfactory as hereinafter described.

A preferred-embodiment of dancer roll according to the invention is shown in Figs. 4 and 5, wherein 22 is a a support stand for the assembly to which a running web is supplied from amill roll, or other appropriate source not shown, disposed to the right of the apparatus as viewed in Fig. 4 and from which the web leaves along line B to downstream processing apparatus as to which substantially constant tension in the web is desired. Idler rolls 23 on the web entrance side and. 24 on the web exit side are provided to obtain a Wrap for contact with dancer roll it journaled at'the ends of the two arms 27 of the link support. As hereinbefore described, dancer roll it) is preferably a thin-walled tube of relatively small outside diameter, such as 2%, 'S.A.E. 1020 steel being a suitable material of construction. Dancer roll it) is provided with short shaft lengths 25 at the ends (Fig. 5), which are attached to the roll by radial struts or other conventional means and which are supported in bearings 26 mounted in the free ends of arms 27, of which there are preferably one at each end of the link support structure. 'Arms 21 are-preferably cut awaycentrally at 28 to reduce the mass. The inner ends of arms 27 are welded to a pivotal mount 30, which may be a heavy wall pipe or similar element having great structural stiffness which is journaled for rotation in pillow blocks 29 bolted to support stand 22. The dancer roll assembly is thus mounted so as to oscillate freely about the longitudinal axis C of mount 30, which therefore corresponds to pivot point 12 of Fig. 2.

Countersupport for the weight of the dancer roll assembly, and the constanttensiou loading which it is de sirable to maintain in the running web, is provided by the vertical air cylinder l7-piston 17 combination, which may be of conventional design, such as that of the Bellofram Corp. hereinbefore referred to, mounted on support stand 22 out of the line of web travel. The upper end of piston rod 17 is provided with clevis 33, as most clearly shown in Fig. 5, and is attached to crank arm 34 integral with mount 30 by loose pin 35 permitting rotational freedom between the elements during oscillation of dancer roll 10.

The dancer roll is used conjointly with short term tension variation removal to supply the speed control for the mill roll unwind motor, which is not shown in the drawings because it is not related to this invention, by the use of the conventional electrical reactor 36 which is in electrical circuit with the unwind motor through leads 37. Reactor 36 may be Reliance Electric & Engineering Co. model #73126 which is provided with an armature piston (not shown) fitted to piston rod 38 and adapted to be moved with respect to the reactor coil as a function of the rotational position of mount 30. The purpose of reactor 36 is to detect the position assumed by mount 30 when the latters position has changed by a certain permissible amount beyond a preselected position inspace and thereupon generate a signal which is passed to the unwind mechanism of the web in process, 7

thereby increasing or decreasing the speed of web sup ply to the equipment to restore dancer roll 10 to the optimum of approximatelyhorizontal disposition in space. To effect this a conventional feedback type electronic speed regulation apparatus in circuit with reactor leads 37 is employed which compares the voltage signal outformed at the base with a downwardly disposed arm 43..

The lower end of arm 43 lies between the two oppositely disposed limit points 44 and 45 of sliding block 46 with sufiicient looseness to permit freedom of movement of roll 10 through a relatively small arc of oscillation, e.g. a 3" rise or fall of the dancer roll with respect to the horizontal as hereinbefore described for a typical installation, to thereby absorb momentary positional shifts of roll 10 due to mill roll out-of-roundness without constantly applying 1 a correctional signal to the unwind motor. Sliding block 46 reciprocates over a small distance on machined way 47 and is, of course, fixedly attached to the outer end of armature piston rod 38 to, by the latters movement with respect to the coil of reactor 36, thereby generate the voltage signal required for comparison with the set point signal of the speed regulation apparatus hereinbefore mentioned. Reactors of the type of 36 have very limited armature travel of the order of A" full range and are practically frictionless, even when the movement of the armature is not precisely axial,so that the apparatus opposes practically zero resistance to free oscillation of dancer roll 10, while at the same time maintaining the dancer roll in a near-horizontal plane as regards long term term frictional drag imposed on the web by the un wind system itself.

From the foregoing it will be understood that this invention may be modified in numerous respects as by the use of a single arm 27, where' sufiicient structural rigidity exists, and in other ways without departure from the essential spirit and it is intended to be limited only by the scope of the following claims.

What is claimed is:

1. A dancer roll assembly comprising in combination an air cylinder, a piston provided with a piston rod reciprocally mounted in said air cylinder, a link support pivoted at one end for rotation about a fixed point pinned to the end of said piston rod opposite said air cylinder, and a cylindrical dancer roll journaled in the free end of said .link support so as to be freely oscillatable arcuately upon movement of said link support, said dancer roll having the form of a thin-wall hollow cylinder with outside radius differing not more than about 30% from the radius of gyration of the subassembly constituting said dancer roll and said link support as an entity.

2. A dancer roll assembly according to claim 1 wherein the web-contacting surface of said dancer roll is provided with a finish such that there is substantially zero slippage between said web-contacting surface and a web in travel over said dancer roll. i 3. A dancer ,roll assembly according to clalm 1 provided with auxiliary means connected in control relationship'with the unwinding system for the web in travel over said dancer roll adapted to regulate the unwinding speed of said web to maintain said dancer roll in substantially the samepreselected position in space.

References Cited in the file of this patent UNITED STATES PATENTS 1,918,968 Keen'ey et al July 18, 1933 2,006,628 Cline July 2, 1935 2,296,753 Wilton Sept. 22, 1942 2,323,818 Lessman July 6, 1943 2,648,504 Ertner Aug. 11, 1953 2,661,773 Kretzschmar Dec. 8, 1953 2,674,110 Roughsedge et al. Apr. 6, 1954 2,750,128 Hittle June 12, 1956 

